1. Field of the Invention
The present invention broadly relates to an improvement in the design and structure of SRAM memory cells, and more particularly, it relates to a novel design and layout of a Data-cache (D-cache) cell that is more symmetric and insensitive to lithography alignments during manufacture.
2. Description of the Prior Art
In the field of microprocessor and cache memory designs, particularly, groups of SRAM cells making up a D-cache that is formed in close proximity to the microprocessor and in which speed of access is critical, it has been found that these D-cache cells were very sensitive to alignments—particularly between the transistor gate conductor regions (“PC”), e.g., of the pull-up transistor, and respective local interconnects (“MC”) connected to drain/source regions of the transistors, and, further, have asymmetries in both schematic and layout. Consequently, production yield has been very low.
For example, FIG. 1 illustrates a circuit schematic 10 of the prior art D-cache comprising an SRAM cell 12 having a group of six (6) transistors, four of which indicated as P0, P1, N4 and N5 are of a cross-coupled latch configuration of typical SRAM design. As shown in FIG. 1, read and write access to the SRAM cell is accomplished via WBL (true side) and WBL (complement side) the respective bit line and bit line complement and are accessed by controlling respective access transistors N12 and N11 and the word line signal (WLW). In order to perform a very fast read (cache write-through) the SRAM cell design 10 further includes additional transistors N16 and N18 forming a first additional read port, and transistors N15 and N17 forming a second additional read port, both connected to the WBL at the intersection of the SRAM P1 and N5 transistors so that data can be quickly accessed and/or replicated.
As shown in FIG. 1, there is an asymmetry from the schematic perspective as no corresponding read port is provided for connection with the WBL line. This asymmetry is critical as it would be desirable on the complement side ( WBL line) comprising transistors P0 and N4 to effortlessly store complementary bit data as it is designed. However, provision of first and second additional read port circuits 14a and 14b comprising read port transistors N16 and N18 provide a leakage path to ground, which has been shown to detrimentally affect operation of the P1 pull-up transistor of the SRAM. In order to keep the cell high on the true side (e.g. logic “1”), P1 has to supply additional currents to overcome the leakage to ground. There is no such leakage path if the complement side is high thus, rendering it harder to maintain a stored “1”.
As further shown in FIG. 2(a), with like reference numbers corresponding to like elements depicted in the circuit schematic of FIG. 1, there additionally exists an asymmetry with respect to the physical cell layout. As shown in FIG. 2(a), in the physical cell layout depicted, there is provided active silicon (“RX”) area 21 in which the physical read port transistors N15-N17 reside; active RX area 31 in which the SRAM pull-up transistors P0 and P1 reside; and active RX area 41 in which the SRAM transistors N4, N5 and bit line transistors N11, N12 reside. The PC polysilicon gates for transistors N15, N16 are shown as the horizontal areas 22, 32, for example, and the PC polysilicon gates for pull-up transistor P0 and transistor N4 and the PC polysilicon gates for pull-up transistor P1 and transistor N5 are shown as the respective horizontal areas 42, 52 in FIG. 2(a). Areas 22, 32 and 42 are connected. As shown in FIG. 2(a), between arrows depicting a distance 30 between the polysilicon gate edge of P1 and the edge of the L-shaped MC (local interconnect) 23. This distance is the minimum allowable according to current lithography groundrules. It is noted that the corresponding distance between the PC polysilicon gate for pull-up transistor P0 and its local interconnect depicted by irregular shaped region MC 33, is increased due to the presence of a notch or jog that effectively increases the space between upper edge of the PC and the lower edge of the MC.
Referring to FIG. 2(b) there is shown the transistors P0 and P1 and the approximate location of respective wide spacer structures 44, 45 as indicated. The spacer structures may be such as described in commonly-owned, co-pending U.S. patent application Ser. No. 10/277,907 (US Publication No. 2004/0075151). As shown in FIG. 2(b), the P1 spacer 45 cuts into the L-shaped local interconnect 23 while on the other side, the P0 spacer 44 is separated from the jogged area of the interconnect 33 without encroachment of the MC. The consequence of this is as follows: the performance of P1 is substantially weakened due to this smaller distance coupled with the read port transistors loading this transistor. This is a problem illustrated in more detail in FIG. 3, which depicts the pFET structure P1, through a cross-sectional view, including the spacer structure 45. As shown in FIG. 3, the MC is depicted as encroaching the spacer 45 and is not separated from it. This is because underneath the MC is a layer of highly-conductive silicide (e.g., CoSi) formed above the drain/source region. However, it is clear that the left corner of the MC 23 encroaches before the end of the CoSi region, i.e., the MC 23 is etched into the spacer and cuts into the silicon closer than the CoSi is located which is detrimental to the device performance as it is closer to the shallow doped drain/source extensions. That is, it has been found that the MC material 23, such as Tungsten or any conductive metal, for example, combines with the metallic surface formed in the shallow extensions and provides a recombination center for holes. This hole recombination mechanism at the source/drain extensions such as shown in FIG. 3, significantly reduces performance of the pFET P0. This is illustrated in FIG. 4, which depicts the performance of the odd wordlines, i.e., depicted in FIG. 3, that includes the P1 transistor and subsequent odd numbered wordlines, etc. where a substantial number of high percent fails occurs on the odd wordlines (e.g., numbered 1, 3, 5 et seq.) due to the spacer encroachment problem.
Moreover, as shown in FIG. 2(b), a further RX contact asymmetry exists which is depicted by a line 34. This asymmetry is shown by a shortening of the RX region 41b above the N11 transistor as compared to the RX region 41a that is extended at N12.
Thus, in sum, the prior art the transistors for CMOS 10S (90 nm technology on SOI) were optimized for performance in the following way: nFETs received a thin spacer in order to ensure short extension regions and thus lower resistance; pFETs received a wide spacer to allow for high activation anneals in spite of larger boron (B) diffusion. As mentioned, there are six (6) transistors in an SRAM cell: 2 pass gates, 2 pulldowns, and 2 pull-ups, the latter being the pFETs. The multiport cell 10 of FIG. 1 includes additional read or write ports and thus more transistors. In this specific case, there are 4 additional nFETs forming two additional read ports. The additional readports allow for fast read operation after the write operation (write through). This cell was designed with two goals: (1) High performance (fast read/write); and, (2) minimum area consumption. However, symmetry was not considered, neither from the schematic, nor from the layout perspective. The asymmetries in design (layout and schematic) combined with non optimized processing steps (MC etch too deep, too close), leads to substantial yield (and reliability) degradation for this cell, while other cells in the same technology yielded as expected. The mechanism for the yield degradation is understood as a hole recombination caused by the presence of the metallic surface (MC) in the lightly doped drain regions (e.g., FIG. 3).
Moreover, in the prior art designs, as the two pull-op and pull-downs are cross-coupled inverters, on each transistor side (e.g., drain region) the connection went up to one metallization (drain region-MC-CA-M1) and then coming back down on the other side through a CA-MC to another transistor. However, on the other side, the cross-coupling was accomplished by providing a large L-shaped MC interconnect that is flat. Thus, there is an asymmetry as one side goes up to a metallization level (M1 processing) and the other side remains flat at a lower level below.
It would thus be highly desirable to provide a SRAM cell design that addresses the performance and minimum area consumption considerations and, that considers the symmetry aspect in both schematic and layout perspectives.
It would thus be highly desirable to provide a SRAM cell design that is insensitive to overlay or misalignment in processing by maximizing the MC-PC distances.
It would thus be highly desirable to provide a SRAM cell design that addresses the asymmetry problems by accomplishing that both connections on all pull-up and pull-down transistors be connected up to the M1 level.